1. Field of the Invention
This invention relates to light-sensitive compositions and, more particularly, it is concerned with photopolymerization initiators which can be used in light-sensitive compositions comprising polymerizable ethylenically unsaturated double bond-containing compounds.
2. Description of the Prior Art
The photopolymerization of ethylenically unsaturated double bond-containing compounds can be initiated by irradiation of these compounds with ultraviolet light and, particularly, with ultraviolet light in the shorter wavelength region. However, a polymerization initiated only by irradiation with light such as ultraviolet light, no matter how strong the applied radiation is, proceeds much more slowly, compared with the case where polymerization takes place in the presence of a certain type of catalyst capable of chemically producing free radicals. In addition, an extremely long time of exposure to ultraviolet light is required for sufficient polymerization of the monomers used, or a light source of the kind which can exert an extremely strong action upon the monomers to be polymerized is necessitated for sufficient and rapid polymerization. Therefore, many attempts have been made to discover materials which can accelerate the polymerization which takes place due to irradiation with light. A number of compounds have consequently been discovered as photosensitizers or photopolymerization initiators which can be used to increase the speed of photopolymerization which results from irradiation with electromagnetic waves. These compounds possess the function that they have the ability to absorb light and produce free radicals capable of initiating the polymerization. However, conventionally known photopolymerization initiators have several practical problems to be solved, though they can initiate the polymerization of certain monomers, for example, ethylenically unsaturated compounds, which is the object of the present invention. For instance, since the photopolymerization initiation capability of most known photopolymerization initiators is suppressed by interaction of oxygen therewith, the photopolymerization reaction must be carried out in the absence of oxygen. However, the various methods which have been proposed for the removal of oxygen have problems in that there are many difficulties when they are put into practice, and much expense is involved.
Benzanthrone, 1,2-benzanthraquinone and the derivatives thereof are already known as photopolymerization initiators as described in, for example, Insha Kogaku, Shinichi Kikuchi and Eiichi Inoue, Ed., Kyoritsu Shuppan, Tokyo for benzanthrone and Journal of Applied Polymer Science, 2 308 (1959), Jaromir Kosar, Light-Sensitive Systems, published by John Wiley and Sons Inc., New York (1965) and Journal of the Society of Photographic Science and Technology of Japan, 28 7 (1965) for 1,2-benzanthraquinone. However, these compounds influence to a lesser extent the polymerization of ethylenically unsaturated compounds dealt with in the present invention, although their sensitizing function in a cross-linking reaction of resins containing light-sensitive groups such as cinnamoyl group is exhibited.